The art of making perfect predictions

27 August 2010
by Eize de Vries

As wind turbine sizes and wind farm capacity continue to increase, the use of taller towers reinforces the importance of accurate wind resource measurements conducted at hub height. The ability to predict an available wind resource at a site is a major step towards achieving the project's expected financial returns and, equally importantly, minimising project-related risks.

The vindicator can sense and calculate wind speed and direction 300 metres in front of the rotor

Traditionally, anemometers on towers were the only measuring devices employed at potential wind farm sites. But more recently Lidar (light detection and ranging) technology has gradually found a place in the market.

Anemometers

Cup anemometers are still commonly used. These precision instruments comprise a vertical axis rotor with three cups that capture the wind. The number of revolutions per minute is registered electronically, giving a measure of prevailing local wind speeds. Anemometers can operate as a single unit with a wind vane for measuring direction, or as two separate instruments in parallel on a shared support structure at similar height.

Other anemometer types include ultrasonic or laser-type stationary devices that detect the shifting of sound or coherent light reflected from the air molecules. Hot-wire anemometers detect the wind speed through minute temperature differences between wires placed on the windward and leeward sides.

An advantage of non-mechanical anemometers is that they are less sensitive to snow and icing because of the absence of rotating parts. Special models with electrically heated shafts and cups may be used in arctic regions.

Dynamic behaviour

Cup-type anemometers have a so-called rotating mass that introduces a flywheel effect (mass moment of inertia), which results in higher wind speed recordings during gusty wind conditions. When both cup anemometers and stationary ultrasonic devices are used by wind turbine suppliers, the turbine software control settings have to be corrected to compensate for the differences in dynamic behaviour.

From the late 1970s to the early 1990s, wind turbines in the 100-450kW range were common and usually came with tower hub heights up to about 50 metres. Available wind resource statistics in many countries are based upon decades of data collection at 10- and 30-metre heights. These values were then typically extrapolated to calculate the measurements at greater hub heights. However, according to many experts, that method worked relatively well up to about only 50 metres. The accuracy was also dependent upon conditions on the site, such as how flat or hilly was the terrain and the number and size of natural and man-made obstacles.

Yield-limiting factors

One expert recalls that average wind speed predictions for 100-metre hub height, extrapolated from 10- to 30-metre measurements performed in relatively flat conditions, proved to be 20-30% lower compared to the actual yields.

The simplified wind resource calculation applied was blamed for those differences — it could not cope with the effects of obstacles as yield-limiting factors. One explanation was that the wind at 100 metres is comparatively stronger as it is less affected by obstacles. A second contributing factor suggested was a better wind quality, or more stable wind conditions, caused by less turbulence at the greater height. An opposite effect may occur in summer months due to thermal effects.

However, some wind experts report mixed experiences with Lidar systems to date. One leading supplier is in the process of conducting comprehensive comparative testing with an anemometer-based system. A spokesman said recently that indicative mean wind-speed results did show a high correlation between figures from the two methods.

One of the key challenges is to translate Lidar-based data into conventionally obtained data. Computer models used at the moment are based upon single-point measurements at specific heights, whereas a main strength of Lidar-based systems is in profiling a complete rotor from top to bottom. And, while anemometers can be calibrated in a wind tunnel, this is not possible for a Lidar-based system, and the development of an alternative method is required.

Some in the industry argue that a number of Lidar product development steps still have to be taken, including verifying measurements taken in turbulent wind conditions with both methods. The company spokesperson strongly believed in a bright future for Lidar-based systems, particularly for sites with complex terrain.

The final test for whether or not Lidar-based products were producing accurate enough data, he added, was if banks would offer wind farm finance on the basis of wind-resource data obtained with a Lidar device.

Facing the wind

Wind turbines need to be continuously steered towards the prevailing wind direction, with a frequency that depends on geographical and environmental conditions. With modern turbines, the control of the blade-pitch angle in response to wind speed is an important function. As part of wind turbine control, the power output is continuously monitored, and an anemometer placed on top of the nacelle to the rear provides additional information on wind-speed fluctuations — being sited behind the rotor, this anemometer records the wind which has slowed substantially compared to the undisturbed flow in front.

A wind-direction sensor device is typically located parallel to the anemometer to signal when rotor re-adjustment is necessary. These conventional controls can be described as reactive, as the system can only respond to a wind gust or a change in wind direction once it has occurred. In other words, there is no lead time left for adjusting the pitch angle and/or rotor orientation to the changing wind conditions in front of the rotor.

In response, laser-based systems that can sense wind speed, direction and variation are undergoing trials (see below). Initial results indicate that turbine performance can be boosted significantly by aligning the rotor with oncoming wind. Additionally, excessive loads on turbine parts could be substantially reduced through early gust detection capabilities.

The Vindicator Laser Wind Sensor is a forward-looking nacelle-mounted wind-sensing system that accurately measures and calculate wind speed and direction up to 300 metres in front of the rotor, says its maker, US firm Catch the Wind. The eggbeater-shaped wind sensing system is being introduced with a standard installed cost of about $150,000.

It is based on three forward-facing laser beams, each reflecting off dust particles in the approaching wind, which change colour. From this sensed colour change, the wind speed and direction are measured, and optimal turbine alignment and blade pitch can be calculated from this input. Being able to measure the approaching wind 300 metres ahead gives a 20-second lead time at 15.6 metres per second (m/s) wind speed.

For offshore applications, the company teamed up with Canadian AXYS Technologies in 2009 to field-test a WindSentinel -— a buoy mounted with a Vindicator, designed to help offshore developers determine wind resources.

Offshore trials

Trials were conducted off Race Rocks Island in British Columbia, Canada, to determine if buoy motions affected wind measurement. The test comprised data collected on a WindSentinel and parallel data collected from a stationary Vindicator 750 metres away on the island. "The buoy worked flawlessly during the trials, with wind speeds that reached more than 22.2 m/s and wave heights over four metres," reports AXYS product development manager Reo Phillips.

Intelligent rotor blades

Rotor blade manufacturer LM Wind Power of Denmark and two other Danish partners are developing a laser-based wind-sensing system that is integrated into the turbine rotor blades and spinner. The partners involved in this development are Danish technical university Risø DTU and sensor specialist NKT Photonics.

Among the objectives is to determine if the laser-based solution will significantly improve wind turbine load control during operation, thereby enhancing the overall reliability of the turbine, its efficiency and operational lifetime.

LM’s long-term strategy is developing intelligent blades that continuously measure approaching wind and either adapt or supply data to the turbine control system. Integrating Lidar technology into these blades is an extension of the company’s previous blade monitoring technology. The product development is named Wind Lidar.

Built-in information

Risø DTU, meanwhile, reported in January that it had completed the world’s first successful test on a turbine with a laser-based anemometer built into the spinner. The results indicate that this system can predict wind direction and gusts as well as turbulence.

The combination of Lidar technology in the rotor blades and the spinner is said to further optimise the system’s overall capability to measure the wind well before it hits the blades.

LM expects that Lidar-enabled intelligent blades will become commercially available by 2014.